Connected: An Internet Encyclopedia
RFC 1583 - A.3.2 The Hello packet

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### A.3.2 The Hello packet

Hello packets are OSPF packet type 1. These packets are sent periodically on all interfaces (including virtual links) in order to establish and maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers.

All routers connected to a common network must agree on certain parameters (Network mask, HelloInterval and RouterDeadInterval). These parameters are included in Hello packets, so that differences can inhibit the forming of neighbor relationships. A detailed explanation of the receive processing for Hello packets is presented in Section 10.5. The sending of Hello packets is covered in Section 9.5.

            0                   1                   2                   3
            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |   Version #   |       1       |         Packet length         |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                          Router ID                            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                           Area ID                             |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |           Checksum            |             AuType            |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                       Authentication                          |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                       Authentication                          |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                        Network Mask                           |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |         HelloInterval         |    Options    |    Rtr Pri    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                     RouterDeadInterval                        |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                      Designated Router                        |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                   Backup Designated Router                    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                          Neighbor                             |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |                              ...                              |

Network mask

  The network mask associated with this interface. For example, if the interface is to a class B network whose third byte is used for subnetting, the network mask is 0xffffff00.

Options

  The optional capabilities supported by the router, as documented in Section A.2.

HelloInterval

  The number of seconds between this router's Hello packets.

Rtr Pri

  This router's Router Priority. Used in (Backup) Designated Router election. If set to 0, the router will be ineligible to become (Backup) Designated Router.

RouterDeadInterval

  The number of seconds before declaring a silent router down.

Designated Router

  The identity of the Designated Router for this network, in the view of the advertising router. The Designated Router is identified here by its IP interface address on the network. Set to 0.0.0.0 if there is no Designated Router.

Backup Designated Router

  The identity of the Backup Designated Router for this network, in the view of the advertising router. The Backup Designated Router is identified here by its IP interface address on the network. Set to 0.0.0.0 if there is no Backup Designated Router.

Neighbor

  The Router IDs of each router from whom valid Hello packets have been seen recently on the network. Recently means in the last RouterDeadInterval seconds.

Next: A.3.3 The Database Description packet

Connected: An Internet Encyclopedia
RFC 1583 - A.3.2 The Hello packet